Suggested additions to permissions docs

to involve the user. This can be either because the API is not sensitive, or because additional

checks exist.

Another benefit of install time permissions is that is becomes very easy to monitor which apps can

access certain APIs. E.g. by checking which apps have the `android.permission.INTERNET` permission

you can list all apps that are allowed to use APIs that can send data to the internet.

#### Defining a permission

Any package can define a permission. For that it simply adds an entry in the manifest file

### Permission restricted components

As [publicly documented](https://developer.android.com/guide/topics/permissions/overview#permission_enforcement)

it is possible to restrict starting an activity/binding to a service by using permission. It is

a common pattern to

it is possible to restrict starting an activity/binding to a service by using permission.

It is a common pattern to

- define a permission in the platform as `signature`

- protect a service in an app by this permission using the `android:permission` attribute of the

This does not work for app-op or runtime permissions as the way to check these permissions is

more complex than install time permissions.

#### End-to-end A service only the system can bind to

Make sure to set the `android:permission` flag for this service. As developers can forget this it is

a good habit to check this before binding to the service. This makes sure that the services are

implemented correctly and no random app can bind to the service.

The result is that the permission is granted only to the system. It is not granted to the service's

package, but this has no negative side-effects.

##### Permission definition

frameworks/base/core/res/AndroidManifest.xml:

```xml

<manifest>

[...]

    <permission android:name="android.permission.BIND_MY_SERVICE"

        android:label="@string/permlab_bindMyService"

        android:description="@string/permdesc_bindMyService"

        android:protectionLevel="signature" />

[...]

</manifest>

```

##### Service definition

Manifest of the service providing the functionality:

```xml

<manifest>

        <service android:name="com.my.ServiceImpl"

                 android:permission="android.permission.BIND_MY_SERVICE">

            <!-- add an intent filter so that the system can find this package -->

            <intent-filter>

                <action android:name="android.my.Service" />

            </intent-filter>

        </service>

</manifest>

```

##### System server code binding to service

```kotlin

val serviceConnections = mutableListOf<ServiceConnection>()

val potentialServices = context.packageManager.queryIntentServicesAsUser(

        Intent("android.my.Service"), GET_SERVICES or GET_META_DATA, userId)

for (val ri in potentialServices) {

    val serviceComponent = ComponentName(ri.serviceInfo.packageName, ri.serviceInfo.name)

    if (android.Manifest.permission.BIND_MY_SERVICE != ri.serviceInfo.permission) {

        Slog.w(TAG, "$serviceComponent is not protected by " +

                "${android.Manifest.permission.BIND_MY_SERVICE}")

        continue

    }

    val newConnection = object : ServiceConnection {

        ...

    }

    val wasBound = context.bindServiceAsUser(Intent().setComponent(serviceComponent),

            serviceConnection, BIND_AUTO_CREATE, UserHandle.of(userId))

    if (wasBound) {

        serviceConnections.add(newConnection)

    }

}

```

## Permissions for system apps

System apps need to integrate deeper with the system than regular apps. Hence they need to be